Production of a layered lithium-manganese-nickel-cobalt oxide material

ABSTRACT

A process for producing a layered lithium-manganese-nickel-cobalt oxide material, includes producing a raw layered lithium-manganese-nickel-cobalt oxide (‘LMNC’) material. Optionally, a dopant capable of enhancing the performance of the layered LMNC material when used as a cathode material in an electrochemical cell, is introduced during the production of the raw LMNC material. Before any annealing of the raw LMNC material is effected, it is subjected to microwave treatment. The resultant treated material is annealed to obtain the layered LMNC material.

This application is a National Stage Application of International PatentApplication No. PCT/IB2015/055023, filed Jul. 3, 2015, which claims thebenefit of, and priority to, South African Patent Application No.2014/04912, filed Jul. 3, 2014, the contents of these applications beingincorporated entirely herein by reference.

THIS INVENTION relates to the production of a layeredlithium-manganese-nickel-cobalt oxide material. It relates in particularto a process for producing a layered lithium-manganese-nickel-cobaltoxide material suitable for use as a cathode material in a lithium-ionelectrochemical cell or battery, and to an electrochemical cellincorporating the layered lithium-manganese-nickel-cobalt oxidematerial.

With the burgeoning world population and the ever increasing demand forenergy, the world faces an energy crisis with fossil fuels beingdepleted and causing global warming. In an effort to keep up with thesedemands, energy conversion technologies, focussing on lithium-ion cellbattery research, for energy storage, are at the forefront.

It is known that layered lithium-manganese-nickel-cobalt oxide materialscan be used as cathode materials in lithium-ion electrochemical cells orbatteries; however, these known materials have limitations when used ascathode materials, including inadequate capacity, rate capability andcapacity retention/cyclability.

It is hence an object of this invention to provide a process forproducing a layered lithium-manganese-nickel-cobalt oxide material whichprovides improved performance when used as a cathode material in alithium-ion electrochemical cell or battery.

Thus, according to a first aspect of the invention, there is provided aprocess for producing a layered lithium-manganese-nickel-cobalt oxidematerial, which includes

-   -   producing a raw layered lithium-manganese-nickel-cobalt oxide        (‘LMNC’) material;    -   optionally, introducing a dopant capable of enhancing the        performance of the layered LMNC material when used as a cathode        material in an electrochemical cell, during the production of        the raw layered LMNC material;    -   before any annealing of the raw layered LMNC material is        effected, subjecting it to microwave treatment, to obtain a        treated material; and    -   annealing the treated material, to obtain the layered LMNC        material.

The process is characterized thereby that the production of the rawlayered LMNC material does not include any heat treatment at atemperature greater than 600° C., preferably not greater than 550° C.,most preferably not greater than about 500° C. In particular, productionof the raw layered LMNC material does not involve heat treatment whichwould result in annealing of the material prior to the microwavetreatment thereof.

The production of the raw layered LMNC material may include heattreatment at a temperature not exceeding 600° C., preferably notexceeding 550° C., typically not exceeding about 500° C.

When no dopant is present, the layered LMNC material may beLi[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂,Li[Ni_(0.33)Mn_(0.33)Co_(0.33)]O₂ (also known as ‘NMC-333’),Li[Ni_(0.4)Mn_(0.4)Co_(0.2)]O₂ (also known as ‘NMC-442’), or the like.Typically, the layered LMNC material isLi[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂.

The raw layered LMNC material may, at least in principle, be produced byany appropriate process or method, such as a modified Pechini method, asol-gel method, a co-precipitation method, or the like. Typically,however, the production of the raw layered LMNC material may be by meansof a modified, one-step, powder-forming Pechini method.

Thus, the production of the raw LMNC material may be effected by

-   -   maintaining a solution comprising a dissolved lithium compound,        a dissolved manganese compound, a dissolved nickel compound, a        dissolved cobalt compound, optionally a dissolved dopant        compound, a hydroxycarboxylic acid, and a polyhydroxy alcohol,        at an elevated temperature T₁, where T₁ is below the boiling        point of the solution, until the solution gels;    -   maintaining the gel at an elevated temperature until it ignites        and burns to form a Li—Mn—Ni—Co—O powder; and    -   calcining the Li—Mn—Ni—Co—O powder to burn off carbon and/or        other impurities present in the powder, thereby to obtain the        raw layered LMNC material.

The process may include forming the solution of the lithium compound,the manganese compound, the nickel compound, the cobalt compound,optionally the dissolved dopant compound, the hydroxycarboxylic acid,and the polyhydroxy alcohol. The forming of the solution may theninclude admixing a solution of the lithium compound dissolved in asolvent, a solution of the manganese compound dissolved in a solvent, asolution of the nickel compound dissolved in a solvent, a solution ofthe cobalt compound dissolved in a solvent, optionally a solution of thedopant compound dissolved in a solvent, the hydroxycarboxylic acid andthe polyhydroxy alcohol.

While the lithium compound, the manganese compound, the nickel compound,the cobalt compound and, optionally, the dopant compound can initiallyeach be in the form of a separate solution in which each is dissolved,they are preferably all dissolved in the same solvent so that a singlesolution containing the dissolved lithium, manganese, nickel, cobalt,optionally dopant, compounds is then admixed with the hydroxycarboxylicacid and the polyhydroxy alcohol. The lithium, manganese, nickel andcobalt compounds are preferably water soluble so that water, preferablydeionized water, can be used as the solvent of the solution. Thus, inparticular, nitrates of lithium, manganese, nickel and cobalt may beused, i.e. LiNO₃, Mn(NO₃)₂ (more particularly Mn(NO₃)₂.4H₂O), Ni(NO₃)₂(more particularly Ni(NO₃)₂.6H₂O) and Co(NO₃)₂ (more particularlyCo(NO₃)₂.6H₂O) may be used. The solution may thus contain the necessarystoichiometric amounts of LiNO₃, Mn(NO₃)₂.4H₂O, Ni(NO₃)₂.6H₂O andCo(NO₃)₂.6H₂O to obtain Li[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂ as theend product.

The dopant, when present, will, as set out hereinbefore, be selected toimprove electrochemical performance, particularly stability, of theresultant layered LMNC material. The dopant may be an element of GroupsIIA, IIIA or IVA of the Periodic Table of Elements. More particularly,the dopant may be aluminium, germanium or magnesium; however, preferablythe dopant is an element of Group IIIA of the Periodic Table ofElements, particularly aluminium. Such dopant compound will thus also bewater soluble, and may be a nitrate of aluminium, e.g. AlN₃O₉.9H₂O. Whenthe dopant is present, the end product will thus be alayeredlithium-manganese-nickel-cobalt-aluminium oxide (‘LMNCA’)material. The end product may then, in particular, beLi[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)Al_(0.02)]O₂.

Preferably, both the dopant introduction and the microwave treatment areeffected.

The metallic compounds, i.e. the lithium, manganese, nickel, cobalt andaluminium compounds, may instead be any other metallic salts (derivedfrom both weak and strong acids such as the sulphates, carbonates,halides, and the acetates) other than the nitrates.

The hydroxycarboxylic acid acts as a reduction agent, and may be citricacid.

The polyhydroxy alcohol may be ethylene glycol or polyethylene glycol.

The citric acid and the ethylene glycol may be present in the solutionin an appropriate molar ratio of citric acid to ethylene glycol, e.g.about 1:4. The citric acid may initially be in the form of a solutionthereof in water, particularly deionized water.

The process may comprise initially heating the mixture of the citricacid solution and the ethylene glycol to the temperature T₁ withstirring; thereafter, the solution of the lithium, manganese, nickel,cobalt and, when present, aluminium nitrates may be added slowly, e.g.dropwise, to the citric acid/ethylene glycol solution.

As set out hereinbefore, T₁ is below the boiling point of the solutioncomprising the solvent for the lithium, manganese, nickel, cobalt, and,when present, aluminium compounds; the dissolved lithium, manganese,nickel, cobalt and, when present, the aluminium, compounds; thehydroxycarboxylic acid and the polyhydroxy alcohol. It is important thatT₁ be below the boiling point of the solution, to prevent prematureevaporation of the solvent and other components of the solution, i.e. toprevent such evaporation before the gel-forming reaction (polymer gelformation) is complete. When the solvent is water as hereinbefore setout, then 90° C.≤T₁<100° C. Preferably, T₁ may then be about 90° C.

The elevated temperature at which the gel is maintained may be T₂, where90° C.≤T₂<100° C. Preferably, T₂ is the same as T₁. In other words,preferably the solution is maintained at T₁ until there has beencomplete or adequate gel formation, and the gel is then maintained at T₁until it ignites and burns to form the Li—Mn—Ni—Co—O powder, or aLi—Mn—Ni—Co—Al—O powder when aluminium is present as a dopant. The timet₁ that the solution must be maintained at T₁ for complete gel formationand evaporation of liquid components of the solution, and that the gelmust be maintained at until it ignites, is dependent on factors such asthe volume of the solution, T₁, etc, but is typically at least 30minutes.

The calcination of the Li-MN-Ni—Co—O or Li—Mn—Ni—Co—Al—O powder may beeffected at a temperature T₃. T₃ will thus be sufficiently high forcarbon and/or other impurities present in the powder to burn off. Thecalcination will thus be effected in a non-reducing atmosphere,preferably in an oxidizing atmosphere. Thus, T₃ may be as low as 300° C.to 350° C. However, more preferred is 400° C.≤T₃<600° C.; typically, T₃is about 500° C. The calcination may be continued for a period of timet₂, with t₂ thus being long enough to achieve burning off of carbonand/or other impurities to a desired degree, and with t₂ also beingdependent on factors such as the quantity of powder, T₃, etc. Thus,preferably, t₂<12 hours; typically, t₂ may be about 6 hours.

The annealing of the treated material or powder may be effected at atemperature T₄. T₄ will thus be sufficiently high to crystallize thepowder. Thus, preferably, 700° C.≤T₄≤900° C. Typically, T₄ may be about700° C. The annealing may be effected for a period of time t₃, with t₃thus being long enough to achieve a desired degree of annealing, i.e. toachieve a desired degree of crystallinity of the powder. Typically, t₃will be less than 12 hours, e.g. about 8 hours.

The microwave treatment may comprise subjecting the calcined powder tomicrowaves (typically at λ=0.12236 m, 600 W) at about 60° C. for between10 and 20 minutes, typically about 15 minutes. The microwave power maybe less than or greater than 600 W.

As also discussed in more detail hereinafter, the inventors surprisinglyfound that by producing [Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂ orLi[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)Al_(0.02)]O₂ using a modified, onestep, powder forming Pechini method, coupled with a microwaveirradiation of the powder, the Mn⁴⁺ content and site disorder can becontrolled, thereby enhancing/maintaining electrochemical performance,e.g. capacity, cyclability, elimination of impurities, etc. It is thusnot necessary to partially substitute Ni and/or Mn and/or Co withmetallic elements such as Ti, Fe, Cr, Ru or Mg to achieve this purpose.The process of the invention is thus characterized thereby that it doesnot include adding to any of the solutions and/or to the powder ametallic element such as Ti, Fe, Cr, Ru or Mg for purposes of partiallyreplacing some of the Ni and/or Mn and/or Co in[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂ orLi[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)Al_(0.02)]O₂. Thus, the endproduct does not contain any Ti, Fe, Cr, Ru or Mg.

According to a second aspect of the invention, there is provided anelectrochemical cell, which includes a cell housing, a cathode, an anodeand an electrolyte in the cell housing, in which the cathode iselectronically insulated from the anode but electrochemically coupledthereto by the electrolyte the cathode comprising the layered LMNCmaterial produced by the process according to the first aspect of theinvention.

The invention will now be described in more detail with reference to thefollowing non-limiting example and accompanying drawings.

IN THE DRAWINGS

FIG. 1 shows FESEM images of (a) LMNC-mic at low magnification, (b)LMNC-mic at high magnification, (c) LMNCA-mic at low magnification and(d) LMNCA-mic at high magnification;

FIG. 2 shows X-Ray Diffraction (XRD) patterns of LMNC-mic(Li[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂) and LMNCA-mic(Li[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)Al_(0.02)]O₂);

FIG. 3 shows X-ray Photoelectron Spectroscopy (XPS) spectra of (a)LMNC-mic and (b) LMNCA-mic, showing the Mn 2p_(3/2) peak;

FIG. 4 shows first charge-discharge profiles of LMNC-mic and LMNCA-mic;

FIG. 5 shows cycle stability of LMNC-mic and LMNCA-mic at C/10 for 50charge discharge cycles;

FIG. 6 shows typical rate capability of LMNC-mic and LMNCA-mic, chargedand discharged at 0.5 C, 1 C, 2 C and 5 C;

FIG. 7 shows cyclic voltammograms of (a) LMNC-mic and (b) LMNCA-micobtained at a scan rate of 0.1 mV·s⁻¹ (first and second cycles);

FIG. 8 comprises comparative Nyquist plots of LMNC-mic and LMNCA-mic (a)before the 1^(st) cycle and (b) after 50 cycles. Data points areexperimental while solid lines are fitted data;

FIG. 9 shows the Electrical Equivalent Circuit (EEC) used to fit theexperimental EIS data; and

FIG. 10 shows plots of Z vs ω^(−1/2) of LMNC-mic and LMNCA-mic.

EXAMPLE

Experimental

Parent Li[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂ (LMNC) and Al dopedLi[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)Al_(0.02)]O₂ (LMNCA) were preparedusing a modified, one-step powder-forming Pechini method. Citric acid(CA), ethylene glycol (EG) and metal (Li, Ni, Co, Al) nitrates were usedas starting materials. The reducing agent, CA (dissolved in deionisedwater) and EG were mixed in a molar ratio of 1:4 (CA:EG) and heated atapproximately 90° C. while constantly stirring for 30 minutes.Stoichiometric amounts of LiNO₃, Ni(NO₃)₂.6H₂O, Co(NO₃)₂.6H₂O,Mn(NO₃)₂.4H₂O and, for the LMNCA material, AlN₃O₉.9H₂O, were dissolvedin deionised water and then introduced, drop-wise, to the reducingsolution comprising the CA and EG. Each solution or sample was thendehydrated into a gel. The gels were kept at a temperature of 90° C.until the solutions spontaneously formed the desired powders. Thepowders (LMNC and LMNCA) were preheated at 500° C. for 6 hours and eachdivided into two batches. The one batch was annealed at 700° C. for 8hours where the temperature was increased at a rate of 10° C./minute,and then left to cool naturally (the electricity supply to the filamentswas switched off after 8 hours) to room temperature (the resultantsamples were designated LMNC and LMNCA respectively). The other half wasirradiated with microwaves (λ=0.12236 m), where the power was increasedat a rate of 60 W per minute to 600 W and irradiated at this power for15 minutes (the temperature of the samples reached a maximum of 60° C.)and then annealed at 700° C. for 8 hours where the temperature wasincreased at a rate of 10° C./minute, and then left to cool naturally(the electricity supply to the filaments was switched off after 8 hours)to room temperature (the resultant samples were designated LMNC-mic andLMNCA-mic respectively).

The structural characterization was done by XRD using a Bruker AXS D8ADVANCE X-ray Diffractometer with Ni-filtered Cu K_(α) radiation(λ=1.5406 Å). The scanning speed was 0.02° per step with a dwell time of5 s for all samples. The sample powders were mounted in PHI 5400 ESCAand PHI 5000 Versaprobe-Scanning ESCA Microprobe vacuum chambers withbase pressures ≤1×10⁻⁸ Torr. XPS was performed on the samples using anon-monochromatic aluminium (Al) K_(α) source (1486.6 eV) and an Almonochromatic K_(α) source (1486.6 eV), respectively. The XPS dataanalysis was performed with the XPS Peak 4.1 program and a Shirleyfunction was used to subtract the background.

Electrochemical measurements were performed in a two-electrode coin cell(LIR-2032) assembled with the LMNC or LMNCA material as the positiveelectrode and lithium metal foil as the negative electrode using aMACCOR series 4000 tester. The cathodes were prepared by coating theslurry of a mixture composed of 80% active material, 10% acetyleneblack, and 10% polyvinylidene fluoride onto cleaned and polishedaluminium foil. Subsequently, the materials were dried at 90° C. undervacuum (˜10⁻¹ Torr) for 24 h. The cells were assembled in anargon-filled MBraun glovebox (O₂, H₂O<0.5 ppm). The electrolyte was 1MLiPF₆ in a mixture of 1:1 (v/v) EC:DMC. A polypropylene film (Celgard2300, Celgard LLC, Charlotte, N.C., USA) was used as the separator.

Results and Discussion

FIG. 1 shows a comparison of the SEM images of LMNC-mic and LMNCA-mic,reflecting that the synthesised materials comprised of nano-sizedparticles with sizes in the 250-300 nm range for the LMNC-mic, and100-200 nm range for the LMNCA-mic. These particle sizes are comparableto those of un-microwaved samples (LMNC and LMNCA).

FIG. 2 shows XRD spectra of LMNC-mic and LMNCA-mic recorded from2θ=10°-90°. All the peaks are attributed to the typical peaks of ahexagonal phase with space group R3m, except for the super lattice peaksfound between 2θ=22°-30° characteristic of the presence of aLi₂MnO₃-type integrated phase, which by contrast adopts the C2/m spacegroup. The intensity ratio of the 101/(006+102) peaks is greater than 2which is an indication that the cation mixing between Li and transitionmetal layers is small. The lattice parameters, a and c, were calculatedusing the Rietveld and least squares methods and are summarized in Table1.

TABLE 1 Structure parameters of the LMNC and LMNCA samples Sample a (Å)c (Å) c/a LMNC-mic 2.852 14.216 4.985 LMNCA-mic 2.846 14.233 5.001

The lattice parameters a and c represent the interlayer metal-metaldistance and the inter-slab distance, respectively. The LMNCA-mic has ahigher c/a ratio compared to the LMNC-mic, indicating a higher cationordering of the LMNCA-mic. The LMNCA also showed a better cationordering compared to the LMNC, but the LMNCA-mic has a superior cationordering. The inventors thus showed that when doping with a minuteamount of Al (x=0.02) that there is almost no change in the a latticeparameter but a slight increase in the c lattice parameter. This isattributed to the increased ionic radius of Al³⁺ (r(Al³⁺)=53.5 pm)compared to the smaller ionic radius of Mn⁴⁺ (r(Mn⁴⁺)=53.0 pm) [11].Also, the increase in the c lattice parameter, in layered materials, isassociated with faster Li diffusion due to the decrease in theactivation energy of Li hopping. Again, the LMNCA-mic shows a greater clattice and thus faster Li diffusion is expected that would result inbetter rate capability compared to the cathode materials LMNC, LMNCA andLMNC-mic. The c/a ratio is an indication of the hexagonal setting, witha larger ratio indicating higher cation ordering. Partial cation mixingis said to occur if the c/a ratio falls below 4.96. The high c/a ratio,well above that required for distortion of the oxygen lattice, clearlyconfirms the formation of the layered structure. These ratios are ingood agreement with known values. It has thus been shown that thesamples that have been treated with microwaves (LMNC-mic and LMNCA-mic)show the highest c/a ratios (4.985 and 5.001 for LMNC-mic and LMNCA-mic,respectively), compared to the un-microwaved samples (LMNC and LMNCA),and thus has the least cation mixing.

FIG. 3 shows the detailed XPS spectra of the Mn 2p_(3/2) peaks of theLMNC-mic (FIG. 3(a)) and LMCA-mic (FIG. 3(b)).

There is a broadening in both the peak widths, an indication that the Mnexist in more than one oxidation state. In order to confirm theoxidation states and to approximate their contribution to the totalpeak, the Mn 2p_(3/2) of the LMNC and the LMNCA peaks were deconvolutedinto two and three peaks, respectively, as this gives the beststatistical fit. The third peak observed from the LMNC-mic is asatellite peak. The obtained binding energy positions and cationdistribution are summarised in Table 2.

TABLE 2 Mn 2p_(3/2) peak positions and cation distribution Bindingenergy position(eV) Cation distribution Sample Mn⁴⁺ Mn³⁺ Mn⁴⁺ Mn³⁺LMNC-mic 642.8 641.8 44.9% 55.1% LMNCA-mic 643.2 641.8 30.8% 69.2%

The binding energy peak positions corresponding to Mn⁴⁺ and Mn³⁺ arecomparable with other known binding energy values. The microwavedcathode materials show a slightly higher oxidation state for Mn than theun-microwaved cathode materials. This increased oxidation state ofmanganese of the microwaved samples could be the reason for theincreased capacity and the better stability. It is known that the rapidtransformation of layered LiMnO₂ to spinel is due to the ease at whichMn³⁺ disproportionates to Mn²⁺ and Mn⁴⁺. This then allows the Mn to moverapidly through tetrahedral sites as Mn²⁺. Mn⁴⁺ however, has a very highactivation energy barrier for diffusion through tetrahedral sites.Therefore layered materials with a higher oxidation state for theirmanganese are expected to be more stable, as seen in this example. Alsothe specific capacity of Li-rich layered cathode materials can becontrolled by controlling the initial ratio of transition metal cations,particularly lithium and manganese ions.

This method of synthesizing Li[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂and the Al doped Li[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)Al_(0.02)]O₂ thusprovides an opportunity to control the Mn oxidation state and thusengineer a cathode material with better properties.

FIG. 4 compares the first charge-discharge profiles of LMNC-mic andLMNCA-mic at a rate of C/10 (i.e., ˜22.5 mA·g⁻¹, note that 1 Ccorresponds to 225 mA·g⁻¹ current density).

Both electrodes show similar charge profiles with a prolonged voltageplateau at ˜4.5 V. The LMNCA-mic showed a higher charge capacity of ˜375mAh·g⁻¹ compared to the LMNC-mic with a charge capacity of ˜270 mAh·g⁻¹.This also applies for un-microwaved samples where the LMNCA had a highercharge capacity than the LMNC. The LMNCA-mic also has a higher firstdischarge capacity of ˜278 mAh·g⁻¹ compared to the LMNC-mic with adischarge capacity of ˜224 mAh·g⁻¹.

FIG. 5 compares the cycle stability at a rate of C/10 of LMNC-mic andLMNCA-mic when charged between 2.0 V and 4.8 V.

During the first 6 cycles these microwave treated samples showedunstable discharge capacities. From the 7^(th) cycle the dischargecapacities started to stabilize. The first 6 discharge capacities forLMNCA-mic and LMNC-mic, varied between ˜270 mA·h·g⁻¹-˜220 mA·h·g−1 and˜265 mA·h·g−1-˜230 mA·h·g−1, respectively. It is worth noting that eventhough the LMNC-mic material has a better capacity at first, theLMNCA-mic shows a higher capacity from the 28th cycle due to its betterstability.

Because Al is electrochemically inactive, the Al doped cathode material(LMNCA-mic) is expected to show a lower discharge capacity. When the Aldoped material (LMNCA-mic) is synthesized by this hybrid microwavesynthesis, the LMNCA-mic shows superior discharge capacity compared tothe LMNC-mic. It is also worth noting that LMNC-mic and LMNCA-mic havebetter stability and discharged capacities compared to the un-microwavedmaterials.

FIG. 6 compares the rate capabilities, at charge and discharge rates of0.5 C, 1 C, 2 C and 5 C, of the LMNC-mic and LMNCA-mic.

With the XRD data showing that LMNCA-mic has a bigger c latticeparameter compared to LMNC-mic it was thus expected that LMNCA-mic wouldhave a better rate capability. As seen from FIG. 6, this is indeed thecase. The c lattice is not the only factor responsible for high ratecapability. The Mn cation state also plays a role—a high concentrationMn³⁺ will increase the electron conductivity and better the ratecapability. The material with the highest Mn³⁺ concentration is also notthe best in terms of rate capability, again as this is not the onlyfactor contributing to better rate capability. Thus it is deduced that agood combination of the two (c lattice and Mn³⁺ concentration) is neededfor high rate capability.

FIG. 7 shows the cyclic voltammetric evolutions of LMNC-mic andLMNCA-mic.

The initial and second cycle of the LMNC-mic and LMNCA-mic isinterrogated using low scan rate cyclic voltammetry (0.1 mV·s⁻¹). Thepeaks at ˜4.7 V, for both LMNC-mic and LMNCA-mic, of their initialcycles disappear with the 2^(nd) cycle showing the irreversible removalof Li₂O. With the higher peak current at ˜4.7 V for the LMNCA-mic, thehigher first charge of the LMNCA-mic compared to the LMNC-mic can beexplained (see FIG. 4). Thus it can be assumed that the microwaveirradiation in the LMNCA-mic causes more oxygen vacancies and thereforeresult in a higher concentration of Li₂O being removed.

Electrochemical impedance spectroscopy (EIS) represents an importanttechnique for evaluating interfacial electrochemistry and the diffusioncoefficient of lithium ion in lithium ion battery materials. Theimpedance spectra for the LMNC-mic and

LMNCA-mic were measured at a potential of 3.5 V. The spectra wererecorded before the 1^(st) cycle and after the 50^(th) cycle. Prior toevery measurement, the cell was relaxed for 1 h. FIG. 8 presents typicalNyquist plots (Z′ vs −Z″) obtained for the LMNC-mic and LMNCA-mic cells.

A high-frequency semicircle and an intermediate-frequency semicirclecomposed into one semi-circle, and low frequency tails were observed.Generally, the high frequency semicircle is related to a passivatingsurface film, the solid-electrolyte interface (SEI). The intermediatefrequency semicircle is ascribed to the resistance to charge-transferprocess at the electrode/electrolyte interface. The low frequency tailis associated with the Li⁺ ion diffusion process in the positiveelectrode.

The EIS spectra were fitted with an equivalent electrical circuit (EEC)shown in FIG. 9. The fitting parameters involve the solution ohmicresistance of the electrode system (R_(s)), solid electrolyte interface(SEI) film resistance (R_(f)), charge transfer resistance (R_(ct)) dueto lithium intercalation/de-intercalation process, the capacitance ofthe surface film (C_(f)) and the interfacial capacitance (C_(Li)), andthe Warburg element (Z_(w)) describing the solid state diffusion oflithium ions inside the active particles, signified by the straightsloping line (˜45°) at the low frequency region.

The EIS parameters obtained for the LMNC-mic and LMNCA-mic aresummarised in Table 3.

TABLE 3 Summary of EIS parameters extracted from the Nyquist plots.Sample R_(s) (Ω) R_(f) (Ω) C_(f) (μF) C_(Li) (μF) R_(ct) (Ω) Z_(w)(×10⁻⁴) Before cycling LMNC-mic 7.0 ± 0.5 22.4 ± 2.2 0.7 ± 0.1 2.4 ± 0.2152.6 ± 5.4 83.5 ± 4.6 LMNCA-mic 9.0 ± 0.6 31.4 ± 2.7 0.5 ± 0.0 2.2 ±0.2 191.6 ± 7.2  80.2 ± 13.2 After 50 cycles LMNC-mic 16.1 ± 1.3  26.7 ±2.5 0.4 ± 0.1 2.5 ± 0.2 206.8 ± 9.0 48.7 ± 2.9 LMNCA-mic 8.1 ± 0.5  6.2± 1.5 1.9 ± 0.5 2.4 ± 0.2  98.2 ± 3.4 146.5 ± 8.7 

The LMNCA-mic shows the same trend as that of LMNCA, where the surfacefilm resistance (R_(f)) and the charge transfer resistance (R_(ct))decrease with cycling. From the R_(ct) and R_(f) values it can bededuced that the SEI film covering the electrode surface is destroyed orreplaced by redox-active active material with cycling, resulting in thedecrease of the whole impedance of the battery cell. It is observedthat, although the LMNCA-mic showed the highest impedance (both R_(f)and R_(ct)) initially, the impedance values dramatically decreased muchlower than the corresponding values for the LMNC-mic. For example, theLMNC-mic gave ˜22Ω (R_(f)) and ˜153Ω (R_(ct)) for the fresh cell and˜27Ω (R_(f)) and ˜207Ω (R_(ct)) after 50 cycles, while the LMNCA-micgave ˜31Ω (R_(f)) and ˜192Ω (R_(ct)) for the fresh cell and ˜6Ω (R_(f))and ˜98Ω (R_(ct)) after the 50^(th) cycle. It is worth noting that theimpedance for the microwaved samples (LMNC-mic and LMNCA-mic) is lowerwhen compared with the un-microwaved samples (LMNC and LMNCA).

The diffusion coefficient (D_(app)) of Lithium ions was calculated bymeans of Equation 1:

$\begin{matrix}{D_{Li} = \frac{2R^{2}T^{2}}{C_{Li}^{\; 2}n^{4}P^{4}A^{2}\sigma^{2}}} & (1)\end{matrix}$

Assuming diffusion coefficients of the oxidised and reduced Li speciesto be equal (D_(ox)=D_(red)=D_(Li)) and equal concentrations(C_(ox)=C_(red)=C_(Li)). D_(Li) is the diffusion coefficient of thelithium ions, R the gas constant, T the absolute temperature, A thegeometric surface area of the cathode, F the Faraday constant, n thenumber of electrons transferred per molecule during oxidation, C_(Li)the lithium concentration in the cathode material and σ is the Warburgfactor obtained from the slope of the real impedance (Z′) vs. thereciprocal square root of the frequency in the low frequency region(ω^(−1/2)) according to Equation 2, and as exemplified in FIG. 10 forEIS.Z _(w)=σ(1−j)ω^(−1/2)  (2)

As expected the calculated diffusion coefficient value for LMNC-mic of1.59×10⁻¹³ cm²·s⁻¹ is inferior to the value for LMNCA-mic of 2.01×10⁻¹³cm²·s⁻¹.

A green Pechini method was thus used to produceLi[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂ andLi[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)Al_(0.02)]O₂ after which thematerial was pre-heated, microwaved and then annealed. This synthesismethod/procedure showed that the oxidation state of the manganese can becontrolled by doping LMNC with Al, thereby increasing the Mn³⁺concentration, and/or microwaved where the Mn⁴⁺ concentration would beincreased. The results showed that the LMNCA-mic outperforms theLMNC-mic and ultimately is superior compared to LMNC, LMNCA andLMNC-mic. In other words, the process of the invention comprises ahybrid synthesis procedure employing conventional annealing andmicrowave radiation, with Li[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂(LMNC-mic) and aluminium-dopedLi[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)Al_(0.02)]O₂ (LMNCA-mic) beingproduced via this synthesis procedure. These cathode materials arecharacterized by SEM showing that they are nano particulate. Thecrystallinity and layeredness were determined via XRD and the latticeparameters were calculated from the data. The XPS measurements showedthat there is a definite change in the oxidation state of the manganesedue to microwave irradiation. The galvanostatic charge-dischargecharacterization showed that the aluminium doped cathode material thathas been microwaved, has a superior discharge capacity and ratecapability. The electrochemical performance of the LMNCA-mic proved tobe superior.

In order to interpret observed microwave effects, two hypotheses can beconsidered. The first assumes the existence of a purely thermal effect(i.e. hot spots and temperature gradient) by dipolar polarization andionic conduction. The second theory assumes that, besides thermaleffects, there are also non-thermal effects like molecular interactionwith the electromagnetic field. It is generally accepted that there area number of characteristics specific to microwaves as agents forpromoting chemical reactions: (i) the quantum energy of microwaves (10-5eV) which is much lower than that of chemical bonds making it improbablefor microwaves to break or weaken bonds within molecules; and (ii) theintensity of electric and magnetic fields that is unable to cause theshift of any chemical reaction equilibrium. However, it is known thatmicrowave heating reveal results different from those obtained withconventional heating.

The approach to this invention was to develop an optimal strategy forproducing a successful and economical synthesis procedure for cathodematerial. The strategy was to make use of microwave radiation in theprimary stage and of conventional annealing in the later stage ofsynthesis. The inclusion of microwave irradiation in the productionprocess enhances the electrochemical performance of both LMNC and LMNCAin terms of capacity, cyclability/capacity retention, and ratecapability. This is due to the Mn⁴⁺ concentration being controlled bythe microwave irradiation and the Al doping.

The invention claimed is:
 1. A process for producing a layeredlithium-manganese-nickel-cobalt oxide material, which includes producinga raw layered lithium-manganese-nickel-cobalt oxide (‘LMNC’) material,producing the raw layered LMNC material including: maintaining asolution comprising a dissolved lithium compound, a dissolved manganesecompound, a dissolved nickel compound, a dissolved cobalt compound,optionally a dissolved dopant compound, a hydroxycarboxylic acid, and apolyhydroxy alcohol, at an elevated temperature T₁, where T₁ is belowthe boiling point of the solution, until the solution gels; maintainingthe gel at an elevated temperature T₂, where 90° C.≤T₂<100° C. and whereT₂ is the same as T₁, so that the solution is effectively maintained atT₁ until there has been complete or adequate gel formation, until thegel ignites and burns to form a Li—Mn—Ni—Co—O powder; and calcining theLi—Mn—Ni—Co—O powder to burn off carbon and/or other impurities presentin the powder, thereby to obtain the raw layered LMNC material;optionally, introducing a dopant capable of enhancing the performance ofthe layered LMNC material when used as a cathode material in anelectrochemical cell, during the production of the raw layered LMNCmaterial; before any annealing of the raw layered LMNC material iseffected, subjecting it to microwave treatment, to obtain a treatedmaterial; and annealing the treated material, to obtain the layered LMNCmaterial.
 2. The process according to claim 1, which is characterizedthereby that the production of the raw layered LMNC material does notinclude any heat treatment at a temperature greater than 600° C.
 3. Theprocess according to claim 1, wherein the production of the raw layeredLMNC material includes heat treatment at a temperature not exceeding600° C.
 4. The process according to claim 1, wherein the layered LMNCmaterial is Li[Li_(0.2)Mn_(0.54)Ni_(0.13)Co_(0.13)]O₂,Li[Ni_(0.33)Mn_(0.33)Co_(0.33)]O₂ or Li[Ni_(0.4)Mn_(0.4)Co_(0.2)]O₂. 5.The process according to claim 1, wherein the dopant is an element ofGroups IIA, IIIA or IVA of the Periodic Table of Elements.
 6. Theprocess according to claim 5, wherein the dopant is aluminium.
 7. Theprocess according to claim 6, wherein the layered LMNC material isLi[Li_(0.2)Mn_(0.52)Ni_(0.13)Co_(0.13)A1 _(0.02)]O₂.
 8. The processaccording to claim 1, wherein the calcination of the Li—Mn—Ni—Co—Opowder is effected at a temperature T₃, where 400° C≤T₃<600° C.
 9. Theprocess according to claim 1, wherein the annealing of the treatedmaterial is effected at a temperature T₄, where 700° C≤T₄≤900° C., tocrystallize the material.
 10. The process according to claim 1, whereinthe microwave treatment comprises subjecting the calcined powder tomicrowaves at about 60° C. for between 10 and 20 minutes.